A modern method of forming integrated circuit devices involves the formation of silicon devices in a thin film of single crystalline silicon material separated from a substrate of silicon material by a layer of buried oxide or insulator. This is commonly referred to as silicon on insulator construction or SOI construction. SOI construction allows for high performance silicon devices due to, among other reasons, the reduction in junction capacitance between diffused areas within the devices and the substrate.
For certain applications, such as r.f., applications, a very thick buried oxide (e.g., .about.10 .mu.m) is desirable. A thicker underlying dielectric not only reduces capacitance but also reduces high frequency power loss, allowing the fabrication of high Q inductors. For high frequency (Ghz range), thicknesses on the order of 10 .mu.m are desired. However, in the past, the thickness of buried oxides has been limited to on the order of 2 .mu.m or less. This is due to the fact that the difference in thermal expansion between the buried oxide and the silicon causes warping. As the thickness of the buried oxide increases, the amount of warping also increases. Accordingly, there is a need for a thicker buried oxide that does not cause unacceptable amounts of warping.
In addition to the problem of warping, thick buried oxide increases the problems of thermal isolation. The thermal conductivity of silicon dioxide is about one tenth that of silicon. The result is higher operating temperatures for ICs on SOI relative to ICs on Bulk silicon. Local heating can also be a problem. Thus, using a thicker buried oxide to achieve benefits of thicker dielectric has been a trade-off with increased thermal isolation. Accordingly, there is a need for increased effective dielectric thickness that does not cause unacceptable amounts of thermal isolation.